Lipid A preparations derived from the lipopolysaccharides of several gram- negative bacteria, as well as chemically defined synthetic lipid A's and their analogs (both glucosamine mono- and disaccharides), were used to establish the chemical structures required for (a) abolishing the expression of suppressor T cell (Ts) function, and (b) the ability of induce polyclonal activation of B cells. Salmonella minnesota R595 lipid A (diphosphoryl lipid A) possesses both of these activities. Decreasing the number of phosphate groups in lipid A from two to one (monophosphoryl lipid A) as well as decreasing the fatty acyl content primarily by removing the residue at the 3-position resulted in a progressive reduction in toxicity; however, these structural modifications did not influence its ability to abolish the expression of Ts function. Reducing the fatty acyl content from five to four (lipid A, precursor IVA or Ia) eliminated its capacity to influence Ts function, but not to induce polyclonal activation of B cells. None of the monosaccharide analogs of lipid A examined influenced the expression of Ts activity, although some were able to activate B cells polyclonally. Thus, in order to be able to abolish the expression of Ts function, lipid A: (a) must be a glucosamine disaccharide; (b) may have either one or two phosphate groups; and (c) must have at least five fatty acyl groups. Also the chain length of the nonhydroxylated fatty acid, as well as the location of acyloxyacyl groups (2' - vs 3'-position) may play an important role. These findings indicate that the chemical structures responsible for the toxicity of lipid A differ from those that influence its capacity to abolish the expression of Ts function and to induce polyclonal activation of B cells.